P0304
Fix within a few daysCylinder 4 Misfire Detected
Code P0304 means your engine's cylinder 4 is misfiring. Common causes: Ignition coil/spark plug (70%), fuel injector (15%), vacuum leak (5%). Drivability: Limit driving, fix within a few days. Repair cost: $50-$250 DIY, $250-$600 professional.
Can I Drive With P0304?
Limit driving, fix within a few days
Essential trips under 10 miles, avoid highways and heavy acceleration. Stop driving if the Check Engine Light flashes.
Stop driving immediately if: Flashing Check Engine Light, loud engine noises, burning smell, rotten egg smell from exhaust, significant power loss or engine shaking.
Symptoms of P0304
- 1Engine runs rough or idles poorly
- 2Loss of engine power or hesitation during acceleration
- 3Engine shaking or vibrating excessively
- 4Check Engine Light is on, possibly flashing
Common Causes of P0304
Parts most likely causing this code, ranked by probability
A faulty ignition coil or worn spark plug on cylinder 4 prevents proper ignition of the fuel-air mixture.
70% of P0304 cases are fixed by replacing this part
A clogged or faulty fuel injector on cylinder 4 fails to deliver the correct amount of fuel, causing a misfire.
15% of P0304 cases are fixed by replacing this part
An air leak near cylinder 4 or in the intake manifold can lean out the fuel mixture, causing a misfire.
5% of P0304 cases are fixed by repairing this leak
Low compression in cylinder 4 due to issues like worn piston rings, a bad valve, or a blown head gasket prevents proper combustion.
5% of P0304 cases are fixed by addressing this mechanical issue
Damaged wiring to the ignition coil or fuel injector, or a rare ECM fault, can disrupt the cylinder's operation.
5% of P0304 cases are fixed by repairing wiring or replacing the ECM
Check These First (Before Buying Parts)
- Visually inspect the spark plug and ignition coil for cylinder 4 for any cracks, corrosion, or signs of damage.
- Listen for any obvious vacuum leaks (hissing sounds) around the intake manifold or vacuum lines near cylinder 4.
- Check the condition of the wiring harness connected to the ignition coil and fuel injector for cylinder 4 for any fraying or damage.
How to Diagnose P0304
- 1Check for a flashing Check Engine Light, which indicates an active, severe misfire.
- 2Visually inspect the ignition coil and spark plug for cylinder 4 for any obvious damage or wear.
- 3Swap the ignition coil from cylinder 4 with an adjacent cylinder (e.g., cylinder 3) and see if the misfire code moves.
- 4Swap the spark plug from cylinder 4 with an adjacent cylinder and see if the misfire code moves.
- 5Listen to the fuel injector on cylinder 4 with a stethoscope to confirm it's clicking, then consider swapping it with an adjacent cylinder if the misfire persists.
- 6Perform a compression test on cylinder 4 to check for internal engine mechanical problems.
- 7Check for vacuum leaks around the intake manifold and vacuum lines using a smoke machine or by listening for hissing sounds.
P0304 on Popular Vehicles
Honda Accord
2003-2012Ignition coil failures are a common cause for P0304 on these models. Spark plugs should also be checked, especially if they are past their service interval.
75% fixed by replacing ignition coil or spark plug
Toyota Camry
2007-2017Similar to Honda, ignition coil and spark plug issues are the primary culprits for P0304. Fuel injector issues are less frequent but do occur.
70% fixed by replacing ignition coil or spark plug
Ford F-150
2004-2010On V8 F-150s, P0304 can be caused by worn spark plugs (especially if original), failing ignition coils, or sometimes a faulty fuel injector. Vacuum leaks are also a known issue.
65% fixed by replacing spark plug or ignition coil
Frequently Asked Questions
What does P0304 mean?
P0304 indicates that your vehicle's engine control module (ECM) has detected a misfire specifically in cylinder number 4. This means cylinder 4 is not contributing power to the engine as it should.
Can I drive with P0304?
Limit driving, fix within a few days. If the Check Engine Light is flashing, stop driving immediately. A flashing light means severe misfires are occurring, which can quickly damage your catalytic converter. If the light is solid, you can drive for essential short trips, but prolonged driving will cause damage.
Will P0304 clear itself?
No, P0304 will not clear itself. The underlying issue causing the misfire must be diagnosed and repaired. Once the repair is complete, you can clear the code with an OBD-II scanner, or it will clear after several drive cycles if the problem is resolved.
How do I fix P0304 myself?
Start by inspecting the spark plug and ignition coil for cylinder 4. Swap the coil with an adjacent cylinder to see if the misfire moves. If not, swap the spark plug. If the misfire persists, check the fuel injector for cylinder 4. A compression test can rule out mechanical engine issues.
How much does it cost to fix P0304?
DIY repair costs range from $20 for a single spark plug to $150 for an ignition coil or $250 for a fuel injector. Professional repair costs typically range from $250 to $600, including diagnosis, parts, and labor, depending on the cause.
P0304 keeps coming back - why?
P0304 keeps coming back because the root cause of the misfire has not been fully resolved. This often happens when only one component (like a spark plug) is replaced, but the actual problem is a different component (like a fuel injector or a vacuum leak), or a deeper mechanical issue.
Will P0304 fail emissions/smog test?
Yes, P0304 will cause your vehicle to fail an emissions or smog test. A misfire indicates incomplete combustion, leading to increased pollutants in the exhaust. The Check Engine Light being on also automatically results in a test failure in most states.
What's the difference between P0304 and P0300?
P0304 specifically indicates a misfire in cylinder number 4. P0300, on the other hand, means a 'Random/Multiple Cylinder Misfire Detected,' indicating misfires are occurring across multiple cylinders or are not consistently tied to a single cylinder.
P0304 on Honda Accord?
P0304 on a Honda Accord, especially models from 2003-2012, is frequently caused by a failing ignition coil or a worn spark plug on cylinder 4. Fuel injectors are less common but still a possibility. Always start diagnostics with the ignition system.
Questions to Ask Your Mechanic
Did you perform a compression test on cylinder 4?
Good answer: Yes, compression is X PSI, which is within specification. This rules out major internal engine damage.
Did you swap the ignition coil and fuel injector to see if the misfire moved?
Good answer: Yes, I swapped the coil and the misfire stayed on cylinder 4, so I then swapped the injector and the misfire still stayed on cylinder 4, indicating the issue is not with those components.
Repair Cost Estimate
DIY Repair
Parts only
$20-$250
Shop Repair
Parts + labor
$250-$600
Save $230-$350 by doing it yourself
Difficulty
intermediate
Time
0.5-2 hours
Intermediate - While replacing a spark plug or ignition coil is beginner-friendly, diagnosing the exact cause of a P0304 misfire can require a scan tool and more advanced diagnostic steps like swapping parts or performing a compression test.
Cost of Waiting
Continued misfires send unburned fuel into your exhaust system, which can severely damage or destroy your catalytic converter. A $50-$250 spark plug or ignition coil job now prevents a $800-$2,500 catalytic converter replacement later.
While You're In There...
If you are replacing the ignition coil, it makes sense to replace the spark plug at the same time as they wear together and require the same labor to access.
If your vehicle uses separate coil boots, replacing them with the spark plug or coil ensures a good seal and prevents future misfires from arcing.
Applying dielectric grease to the spark plug boot helps prevent moisture intrusion and ensures a good electrical connection, preventing future misfires.
Tools You'll Need
- OBD-II Scan Tool
- Basic Hand Tools (ratchet, sockets, extensions)
- Spark Plug Socket
- Torque Wrench
- Multimeter (for electrical checks)
- Compression Tester (if needed)
P0304 is a very common misfire code. Most repairs are straightforward, often involving a simple spark plug or ignition coil replacement, and your engine will run smoothly again afterward.
- Family-owned since 1957
- 990,000+ happy customers
- Fast nationwide shipping
- Easy returns
Your Next Steps
Step 1: Visually inspect the ignition coil and spark plug for cylinder 4. Look for cracks, corrosion, or signs of oil fouling.
Step 2: Swap the ignition coil from cylinder 4 with an adjacent cylinder. Clear the code and drive the vehicle. If the misfire moves to the new cylinder, replace the ignition coil. If it stays on cylinder 4, swap the spark plug.
Step 3: If swapping the coil and plug does not resolve the issue, consider checking the fuel injector for cylinder 4. You may need to perform a compression test to rule out mechanical engine problems. Parts you'll likely need: a single ignition coil or a single spark plug.